The present invention relates to a method for the production of olefins and particularly to processing the cracking heater effluent to more effectively recover the product and process the by-products.
In the production of ethylene and propylene through the pyrolysis of a variety of feedstocks, several byproducts and unsaturated diolefins and acetylenes are created. The net effluent from the pyrolysis heaters, typically referred to as charge gas, requires processing for the separation of the byproducts and removal of the diolefins and acetylenes from the primary olefin products. Removal of the C2 and heavier diolefins and acetylenics from the cracked gas is handled through a combination of separation via distillation and reaction via hydrogenation. Specifically for acetylene, separation alone would result in excessive loss of the ethylene product since acetylene and ethylene have very similar relative volatility. Currently, the distillation and hydrogenation take place in several distinct process steps that are designed to separate and hydrogenate the C2, C3, and C4 compounds independently. Separation of the different hydrocarbons before hydrogenation is currently required for achieving better control over the hydrogenation, prolonging catalyst life, and enhancing performance.
One disadvantage of this widely practiced conventional technology is the large energy consumption necessary to generate the high pressures and cryogenic temperatures required to separate first the hydrogen from the cracked gas and then subsequently the molecules of higher carbon number. Additionally, the hydrogenation steps for each of the hydrocarbon groups require an independent reactor system consisting of several pieces of equipment driving up the capital investment and complexity of the plant.
The invention outlined in the previous U.S. Pat. No. 5,679,241 proposes the one-step conversion of all C2 to C5 and heavier acetylenes and dienes without hydrogenation of the C2 or C3 olefins. It is claimed that this is possible with one catalytic distillation unit capable of treating the hot, relatively low pressure charge gas before excessive compression and cryogenic cooling is performed. In addition, if desired, this same one step process claims to be capable of hydrogenating the C4 olefins to paraffins again without the loss of C2 or C3 olefins. The patent relates to a system that is described as being capable of removing 70% and more of the hydrogen in the cracked gas prior to the required cryogenic separation by the hydrogenation of the C2 to C4 acetylenes and dienes and the C4 and heavier olefins to paraffins. Removal of 70% or more of the hydrogen improves the economics through a significant lowering of the energy requirements for separation of the C2 and heavier components. By reducing the hydrogen partial pressure, separation is achieved at lower pressures and with reduced refrigeration. However, it has been shown that such extensive hydrogenation in a single step system cannot occur without substantial loss of ethylene and propylene to paraffins by hydrogenation.
The process as described in U.S. Pat. No. 5,679,241 has significant limitations. First, in the operation of an ethylene plant, the C2 acetylene specifically must be removed via hydrogenation since its removal via distillation is extremely difficult requiring extensive equipment and energy costs. Since acetylene is a polymerization catalyst poison, it must be removed to low levels, often less than 1-2 ppm. The ability to hydrogenate all of the C2 acetylene to that level in a single catalytic distillation column while observing no ethylene loss or preferably a gain was not possible at reasonable catalyst volumes and commercially viable operating conditions. Second, maintaining performance of the process during either a variation in carbon monoxide flow, which impacts catalyst activity, and/or the concentration of diene/acetylene in the feed was hard to manage and would prove difficult to achieve at commercial scale. Third, methods of handling eventual catalyst deactivation were limited. Since these units must operate for long periods of time between shutdowns, the only options were excessive catalyst or separate catalyst zones in the reaction column that can be isolated and the catalyst replaced while the other section remains in operation. When using larger catalyst volumes, it is known that it is necessary to operate at lower temperatures to avoid over-reaction while the catalyst is still active. This negatively impacts the economics by requiring some refrigeration to control operation at lower temperature and/or excessive recycles of cool liquid within the column. Specifically pilot testing has shown that:
a. When the single catalytic distillation column was operated to remove greater: than 95% of the C2 acetylene, the concurrent ethylene loss was above 1% by weight. This is undesirable economically.
b. When operating a single catalytic distillation column, if the hydrogenation of C4 olefins is greater than 20%, significant ethylene loss is to be expected with presently available catalysts.
c. In order to achieve minimal ethylene and propylene losses while operating a single catalytic distillation column and maintaining extremely high conversions and hydrogen removals, the design required excessive catalyst as evidenced by low productivity and operation at cooler temperatures requiring refrigeration.
d. A significant variation in catalyst activity will occur with variations in the carbon monoxide in the feed. Such variations if seen in a single step process, will result in loss of acetylene removal efficiency and subsequent products which do not meet specification. This impact on performance due to the loss of catalytic activity via CO poisoning is equivalent to the impact on performance due to catalyst aging.
e. A significant variation in feedstock to the ethylene cracking heaters will result in substantial changes in both the acetylenes and dienes as well as the hydrogen flow. As the ratio of hydrogen to reactants changes, a single step process has limited ability to follow such changes. The results will be either a breakthrough of acetylene leading to offspec ethylene product or a high loss of valuable ethylene and propylene due to over-reaction, unless the system has substantial and expensive overdesign which could be utilized to surmount these process changes.
The present invention relates to an improved process for the processing of the charge gas effluent from the pyrolysis of a variety of feedstocks. The primary objective is still to remove a significant fraction of the hydrogen in the effluent by hydrogenating the C2 to C5 diolefins and acetylenes in the feed while achieving essentially total hydrogenation of the C2 acetylene without significant hydrogenation of the ethylene and propylene. In the improved process, this is achieved even with disturbances in the carbon monoxide concentration, varying diene and acetylenic feed concentrations and catalyst deactivation as well as other foreseeable processing upsets. The invention relates to catalytic distillation with improved liquid recycle in combination with fixed bed hydrogenation reactor systems. Specifically, the operating conditions of the catalytic distillation are maintained or adjusted to obtain the maximum hydrogenation of the acetylenes and dienes but without any loss of ethylene and propylene and preferably with an ethylene gain by C2 acetylene hydrogenation. Maintaining a stable high conversion of all of the C2 to C5 acetylenes and dienes with 100% conversion of the C2 acetylene (still without hydrogenating the ethylene and propylene) under varying process conditions is made possible by the fixed bed hydrogenation system in which the remaining C2 acetylene is completely hydrogenated again without significant ethylene or propylene hydrogenation.